When most people think of cardio, the first image that comes to mind is someone walking on a treadmill day in, day out, every day. To be honest, if I had to do that I think I'd go insane. It may sound hard to believe, but in all these years (actually decades) of working out I've never done that. There are so many better options, why would I? First, what is cardio and what is the purpose of it? It's really quite simple. It's a workout for your heart and circulatory system. By raising your heart rate for a sustained period…